1. Field of the Invention
This invention relates to a process improved in several ways for the production of surface-active or interfacially active water-soluble, water-emulsifiable or water-dispersible anionic surfactant salts which may be used, for example, as useful materials in wetting agents, detergents and/or cleaning products. In the interests of simplicity, anionic surfactant salts of the type in question are referred to hereinafter as washing-active anionic surfactants or, quite simply, as anionic surfactants.
Washing-active anionic surfactants are important principal components or mixture components in any applications where interfacially active or surface-active auxiliaries are used. The anionic surfactants are generally used in admixture with other types of surfactants, more particularly anionic surfactants.
It is known that the class of anionic surfactants encompasses compounds in which hydrophobic hydrocarbon radicals are attached to hydrophilic, negatively charged groups which, in turn, are capable of salt formation. The most important representatives of the hydrophilic negatively charged groups are carboxyl groups and/or the residues of inorganic acids, more particularly sulfuric acid and/or phosphoric acid. The associated anionic surfactant classes include the carboxylates or soaps, the sulfates or the sulfonates, the phosphates or the phosphonates. In the numerous fields of application involving anionic surfactants of this type, overriding significance is attributed in particular to the sulfonates and sulfates in addition to the soaps. However, the effect of the anionic surfactants in practical application is also critically determined by the choice of the optimal salt-forming cations. In general, desirable representatives of these cations are those which lead to water-soluble or at least sufficiently water-emulsifiable or water-dispersible anionic surfactants. Overriding significance is again attributed here to the salts of the alkali metals, more particularly sodium and/or potassium, to the corresponding ammonium salts and/or to salts based on organically substituted ammonium ions. Accordingly, the synthesis of the anionic surfactant salts includes the neutralization--often as the last step--of the anionic surfactant intermediates still bearing acidic groups with suitable salt-forming inorganic and/or organic bases. Aqueous preparations of the bases, for example aqueous sodium hydroxide, which introduce water into the reaction product through their water content are frequently used for this purpose. Additional water is formed as a product of the salt-forming neutralization reaction. Accordingly, to obtain the anionic surfactant salts in dry form, subsequent drying is generally necessary. In addition, aqueous components in the anionic surfactant salt reaction product can be of different origin. Thus, the primary reaction products from the organic basic components and the associated acids, particularly inorganic acids, often have to be bleached in aqueous phase to obtain surfactants sufficiently light in color. Accordingly, the intermediate production of aqueous concentrates, which may often be present as aqueous pastes, is standard practice in the manufacture of modern anionic surfactants.
The conversion of these aqueous preparations of the anionic surfactants into powder-form or granulated dry products on an industrial scale is carried out by spray drying to form fine powders which can be agglomerated into relatively large agglomerates in a separate process step or even in a subsequent process step integrated into the drying stage. All this is generally known among experts.
Using this drying technology, one elegant solution to the problem of salt formation in the final stage of the synthesis of anionic surfactants is based on the principle of so-called spray neutralization. In this process, the two steps of neutralization using, above all, water-containing reactants both on the side of the anionic surfactant intermediates and on the side of the cation-yielding base components may be combined into a single step with the subsequent drying of the anionic surfactant salts formed. The components to be reacted with one another are sprayed and are mixed together in this form. The material accumulating may be delivered to a typical spray drying zone filled with hot gas, so that the salt-forming neutralization reaction and removal of the water used as auxiliary medium and the water formed by the neutralization reaction can ultimately be carried out in a single integral step.
The spray drying of aqueous preparations of useful materials of the type mentioned which are used on a large scale, for example as constituents of laundry detergents, has been carried out on an industrial scale for decades. Hot air and mixtures of air and hot waste combustion gases are used as the drying gas stream. Washing powders or useful materials and/or mixtures of useful materials for the production of laundry detergents in pourable and free-flowing powder form are obtained in corresponding spray-drying towers, generally at ambient pressure, in co-current or more frequently in countercurrent. Hitherto, the spray neutralization step with which the present invention is particularly concerned has been carried out in the same medium.
2. Discussion of Related Art
In their earlier application DE-A 40 30 688, applicants describe for the first time the use of superheated steam as the drying hot gas stream in the production of powder-form and, optionally, agglomerated useful materials or mixtures of useful materials for wetting agents, detergents and/or cleaning products of aqueous preparations thereof. Particular emphasis is also placed in this document on the corresponding drying of washing-active anionic surfactant salts with superheated steam. This new technology of drying components of wetting agents, detergents and/or cleaning products with superheated steam is further developed and optimized in a number of other applications, namely in applicants' earlier German patent applications P 42 04 035.3, P 42 04 090.6, P 42 06 050.8, P 42 06 521.6 and P 42 06 495.3. The disclosure of these earlier patents and patent applications is specifically included herein as part of the disclosure of the present invention.
The teaching according to the present invention as described hereinafter is based on applicants' knowledge of the drying of useful materials and mixtures of useful materials of the type in question with superheated steam as the drying gas, but at the same time goes a crucial step further which, to applicants' knowledge, has never been reported in the relevant literature. The essence of the further development according to the invention lies in the following: The inert conditions of a drying zone filled with superheated steam are now also to be utilized for the final process step involved in the production of the anionic surfactant salts. More particularly, the invention seeks to carry out the reaction of the anionic surfactant intermediates still bearing acidic groups with the salt-forming bases, more particularly aqueous gases, of inorganic and/or organic character under the protection afforded by the inert conditions of the zone filled with superheated steam. Accordingly, a greater function is assigned to the medium of the superheated steam, namely: the superheated steam serves as an inert reaction medium for the neutralization reaction. This does not in any way affect its suitability as transport medium for the drying process.